1. Field of the Invention:
This invention relates to a charge transfer device such as a charge coupled device and, more particularly, to an improvement of its output portion for producing a noise-free output.
2. Description of the Prior Art:
Among charge transfer devices, charge coupled devices (CCDs) have been put into practical use in facsimiles, TV cameras, image scaners, and so on. The applicable field of those CCD's are now spreading. There was, however, a drawback in an output structure in which output noises based on the structure were inevitable.
The output structure of CCDs in the prior art is shown in FIGS. 1(a) and 1(b) and its operation will be explained with reference to FIG. 2 and FIGS. 3(a) to 3(c). In a P-type silicon substrate 1, an N-type channel region 2, an N.sup.+ -type output floating zone 11, an N-type reset-channel region 7, an N.sup.+ -type reset-drain region 5 and a P.sup.+ -type channel stopper region 9 are formed. The channel region 2 is composed of a buried-channel type charge transfer register together with a plurality of transfer gate electrodes 31-34 formed in a line on the channel region 2 via a thin insulator film (not shown in FIGS. 1(a) and 1(b)). Here, a constant D.C. voltage is applied to the transfer gate electrode 35. By applying four-phase clock pulses to the transfer gate electrodes 31 to 34, charges are transferred from left to right through a burried channel under the transfer gate electrodes 31 to 35.
At the right end of the channel region 2, the N.sup.+ -type output floating zone 11 is formed to receive charges transferred through the channel region 2. The voltage at the output floating zone 11 changes in accordance with the quantity of received charges and is amplified to be derived as an output signal by an output amplifier 10. After the output signal is derived, the charges at the output floating zone 11 is drained by making the reset-channel region 7 between the output floating zone 11 and the reset-drain region 5 conductive. For making the reset-channel region 7 conductive, a reset pulse is applied to a reset electrode 8. The output floating zone 11 receives next charges to produce a next output, after the charge drain of the output floating zone 8.
The above-mentioned output operation will be explained in a little more detail. In FIG. 2, (a), (b), (c) and (d) respectively show a clock pulse applied to the transfer gate electrode 34, a D.C. voltage applied to the final stage transfer gate electrode 35, a reset pulse applied to the reset electrode 8 and a constant voltage applied at the reset-drain region 5. FIG. 3(a) is a schematic diagram of FIG. 1(b), FIGS. 3(b) and 3(c) being potential diagrams at times t.sub.1 and t.sub.3 (see FIG. 2) shown in relation to FIG. 3(a). The read-out operation is performed as one period from a time T.sub.1 to a time T.sub.5. The charge-drain for resetting the output floating zone 11 is performed within a time duration from T.sub.1 to T.sub.4. Namely, the reset pulse is applied to the reset electrode 8 at T.sub.2 to make a surface portion of the reset-channel region 7 conductive. Under this condition, charges at the output floating zone 11 are drained to the reset-drain region 5. At time T.sub.3, the reset pulse disappears from the reset electrode 9, to disconnect the output floating zone 11 from the reset-drain region 5. Thereafter, the clock pulse at the transfer gate electrode 34 lowers to transfer charges thereunder to the output floating zone 11 (see FIG. 3(c)). The output amplifier 10 detects the voltage at the output floating zone 11 generated by the transferred charges. Thus described output operation is repeated to detect all the quantity of charges transferred sequentially through the transfer channel region 2.
The noise inherent to the output structure is a reset noise generated at the reset operation by the reset pulse. There is a capacitance C between the reset electrode 8 and the output floating zone 11. This capacitance is charged by the reset pulse applied to the reset electrode 8. The stored charges determines the voltage at the output floating zone 11 after the reset pulse disappears. This voltage due to the stored charges appears on the output signal as a noise. The value of the stored charges is a function of an absolute temperature T as shown in an equation of a number of equivalent noise charges (i.e. a Nyquist noise) of ##EQU1## where k is a Boltzmann's constant and q is an electron charge. This is one kind of thermal noises and is described on pages 1 to 12 in IEEE Journal of Solid-State Circuits, Vol. SC-9 (February 1974).
This noise deteriorates a quality of reproduced picture when the output structure is embodied in an image sensor to sense an image having a low illumination intensity. However, the noise may be removed if the output floating zone 11 is made depleted completely to remove all the charges therein just after the reset pulse disappears. This is difficult in the output structure in the prior art, because the impurity concentration of the output floating zone 11 cannot be made such lower value that a complete depletion is possible and has to maintain a value enough for an ohmic connection with wirings for the output amplifier 10. The impurity concentration required for the ohmic connection is more than 1.times.10.sup.20 cm.sup.-3 which is too deep to be made depleted completely.